It is known that wafers, mirrors, glass surfaces and printed circuit boards may be scanned optically by various methods. In these methods the object to be scanned, the wafer, mirror or the glass surface, is scanned by an optical line sensor, involving movement of the line sensor relative to the object, which generates a two-dimensional image. The line sensor is a colour sensor. The line on the surface of the object observed by the line sensor should be illuminated from different angles by light-emitting diodes, similarly arranged in lines, in the basic colours of the sensor, namely red, blue and green. Here at least one colour is used as bright-field illumination and another colour as dark-field illumination.
Bright-field illumination means that the light source on the surface of the object is reflected in the sensor. With dark-field illumination, there is no reflection of the light source relative to the sensor.
In the illumination of objects with reflecting surfaces, on the one hand the light source should lie as far as possible outside the sharp-focus range or focal plane of the sensor, i.e. the greatest possible distance should be maintained between the light source and the surface of the object. On the other hand, though, there should be a high level of light intensity on the surface of the object, for which reason it would be expedient to keep the distance between the light source and the surface of the object as low as possible. Moreover the object should be illuminated from different angles, so that any defect found is shown as clearly as possible. This is very difficult with the conventional light sources for illuminating a line, which generally consist of a light-emitting diode mounted along a line. These conflicting requirements, whereby the distance between the light source and the surface of the object should be on the one hand great and on the other hand small, are especially marked in the case of dark-field illumination of reflecting surfaces, where only a small fraction of the light-beam bundle directed on to the line to be scanned on the surface of the object is captured by the sensor.
In order to meet the requirements described above, an arrangement (FIG. 2) has been used to date in which, to illuminate a plane-surface object 1, one or more linear light sources 2 for dark-field illumination and a linear light source 3 for bright-field illumination are used. The light-beam bundle of the light source 3 for bright-field illumination is directed on to the line to be scanned on the surface of the object, from where it is reflected on to a mirror 4, from which the light-beam bundle is deflected to a line sensor 5. Typically used as line sensor is a one-dimensional colour CCD element with optics mounted in front. It is however also possible to use a camera which generates a two-dimensional image, of which only a single line is analysed. The light sources 2 for dark-field illumination are arranged very close to the surface of the object, so that a sufficiently high level of illumination is obtained.
However this has the consequence that the individual light-emitting diodes from which these linear light sources are formed are perceived by the camera as individual spots of light, since these light sources are located in the sharp-focus range of the sensor. Uniform and homogeneous illumination of the line to be scanned would be desirable, but is not possible with this arrangement of the dark-field illumination.
A further disadvantage lies in the fact that the dark-field illumination provides light only from the direction of the light sources. If the line to be scanned needs to be illuminated from a wider angular range, then either the luminous surface of the light source must be enlarged considerably, or else more light sources must be used.
Patent application US 2005/0001900 A1 discloses a device for the inspection of a wafer, comprising at least two incident illumination devices which emit an illuminating light beam which falls at an angle on the surface of a wafer to be inspected, determining in each case an illumination axis. An image recording device to record an image of the surface is provided in a dark-field array. Also provided is a wafer holding fixture to hold a wafer in a preset alignment. Formed on the surface of the wafer are linear structures. The device is characterised by the fact that the illumination axes are aligned at right-angles to one another and the device is so designed that a projection of the respective illumination axis is directed on to the surface of the wafer, substantially at right-angles to the respective linear structures on the surface of the wafer. The device may be used to detect macro-defects and to evaluate the quality of the edge profile of structures formed on the surface of the wafer.
Patent application US 2005/0002023 A1 discloses a device for the inspection of a wafer, comprising at least one incident illumination device which emits an illuminating light beam which falls at an angle on the surface of a wafer to be inspected, and an image recording device to record an image of the surface in a dark-field array. The wafer inspection device is distinguished by the fact that at least one deflecting device is provided to deflect an assigned illumination light beam on to the surface of the wafer.
Patent application US 2005/0259245 A1 discloses a device and a method for the inspection of an object, with a bright-field illumination beam path of a bright-field light source formed with reference to imaging optics, with a dark-field illumination beam path of a dark-field light source formed with reference to imaging optics, wherein the object with the imaging optics may be imaged on one or more detectors and wherein the object is illuminated simultaneously by both light sources. The device is suitable for simultaneous detection of bright-field and dark-field images.
Patent application WO 2001/023869 A1 discloses a device and a method for the finding and categorisation of surface defects of a moving belt material with an illumination device for generating an illumination surface (inspection surface). A camera set-up for recording an image of an inspection surface is provided, and the camera set-up is linked to a computing and control unit. At least one matrix camera is provided, for dividing the inspection surface image into at least two image sections for separate evaluation.
European patent EP 1 150 154 B1 relates to an arrangement and a method for illumination. Intended here in particular is incident illumination for microscopes, comprising a ring support—oriented around the optical axis—for holding means of illumination. According to the invention the illumination means are light-emitting semiconductor diodes (LEDs) mounted in the ring support, wherein the main direction of radiation of the semiconductor diodes, which have a relatively small angle of departure, is directed or inclined towards the optical axis of the system. The diodes used are white-light diodes, preferably fixed in at least two concentric rows of rings within the ring support. Adjacent LEDs are connected into groups and are operated through a controllable constant current source.
Known from European patent EP 0 426 166 B1 is an illumination system in which three tungsten-halogen filament lamps are used to illuminate a linear section of an object. Two lamps are located close to the surface of the object, and are arranged parallel to the surface. Each of these two lamps is assigned a channel-shaped reflector with elliptical cross-section, by which the linear lamps are focussed on the linear area to be illuminated. The third lamp is arranged at a greater distance from the object. It is focussed on to the linear area to be illuminated by means of two reflectors. Here too, one of the two reflectors has an elliptical cross-section.
European patent EP 0 624 787 B1 shows a device for the surface inspection of silicon wafers. Here the wafer is illuminated at specific points. The light reflected from the wafer is deflected on to a photo-detector by means of an elliptical mirror, rotation-symmetric around an optical axis.